This invention relates to optical communication systems and more particularly to optical communication systems that transmit bipolar digital signals by translating each digital level in the bipolar signal into two bits of a binary signal.
Large numbers of messages are now transmitted over the telephone plant by means of T1 and T2 Carrier Systems. In these systems speech signals are converted into bipolar electrical signals which are essentially binary in nature but the adjacent logic "1s" are caused to alternate in polarity. This alteration in polarity was deemed necessary in order to insure that a sufficient number of transitions would be available in the signal in order to permit the repeaters to derive timing information and to provide dc balance to preclude baseline wander in the ac coupled receiver. In addition, violations in the alternating polarity, known to those in the art as "bipolar violations", are inserted in order to stress receivers by introducing known amounts of baseline wander. The medium used for connecting the terminal stations and repeaters is wire pair cable. A large number of wire pair cables utilized to transmit both 1 and T2 carrier signals have already been installed in the major cities. These cables are physically positioned within ducts beneath the surface of the streets of the cities. Many of the ducts have already been loaded with wire pair cables to their full capacity. Expansion of the telephone plant in these areas, if that expansion is to take place with similar T Carrier Systems will require the installation of additional ducts. It would be advantageous if the existing wire pair cables in these ducts could be replaced by optical fibers inasmuch as each fiber is smaller than a wire and, in addition, may allow larger bandwidths to be transmitted.
In the period of transition when wire pair cables are being replaced by optical fibers, many electrical bipolar signals of the type generated in the T1 Digital Transmission System will have to be converted into optical signals in order to permit them to be transmitted over optical fibers. Inasmuch as there is no straightforward equivalent of two polarities in the optical signal, some sort of conversion is necessary. It would also be advantageous if the resulting optical signal were simply of the binary type as opposed to a multilevel optical signal, in order to simplify the repeater units which will be necessary in the optical transmission system. Finally, it is desirable to maintain the polarity information present in the bipolar signal of the T Carrier System inasmuch as polarity transitions and bipolar violations will continue to provide information to T carrier type equipment operating at the receiving end of the optical transmission system.
One such encoding which will both develop a binary signal in an optical transmission system and preserve the bipolar information is disclosed in a copending application by Messrs. J. S. Cook and S. D. Personick entitled, "Optical Communication System with Bipolar Input Signal" filed Aug. 1, 1975, Serial No. 601,049. In accordance with the Cook-Personick invention, each pulse of the bipolar signal is converted into two binary digits which are then utilized to modulate an optical signal source. Each positive pulse of the bipolar signal is converted into two equal binary digits of a first logic state and each negative pulse of bipolar signal is converted into two equal binary digits of the opposite logic state. Each digital zero or zero voltage level in the bipolar signal is converted into two binary digits of opposite logic states. One feature of this type of conversion is that the two opposite binary digits that are not utilized to represent the digital zero are not generated as a pair in the conversion process. This particular pair of binary digits is in essence a forbidden word with respect to the conversion. In the decoder apparatus disclosed in the Cook-Personick application the binary signal after being detected at the receiving location is stored in a 3-cell shift register. The logic apparatus connected to this shift register is designed to detect the presence of the forbidden code in each of the two pairs of adjacent cells in the 3-cell shift register. The remainder of the decoding apparatus is connected to decode two of the three bits present in the 3-cell shift register. Upon detection of the forbidden word in the two cells being utilized for decoding, the decoding apparatus is switched to the other pair of cells in the 3-cell register. In this way no information is lost as a result of the detection of an out-of-frame condition. Unfortunately, the Cook-Personick approach to word synchronization or framing has the potential shortcoming that an error in the data can be interpreted as an out-of-frame condition thereby causing a reframing which in turn leads to detection on the wrong pair of bits and the introduction of additional errors. In short, this prior art technique of reframing has been determined to be much too sensitive to single transmission errors.